Continuously tunable thin film laser employing the electric field effect

ABSTRACT

A selectively tunable thin film laser comprises a source of excitation energy and a thin film of laser material positioned to intercept the excitation energy. Conductive means are positioned on either side of the laser material and insulated therefrom; a source of selectively variable electrical potential is connected to the conductive means to generate a variable electric field therebetween so as to tunably change the wavelength of the laser output.

United States Patent Inventor Vern N. Smiley San Diego, Calif.

Appl. No. 842,915

Filed July 18, 1969 Patented Apr. 6, 1971 Assignee The United States ofAmerica as represented by the Secretary of the Navy CONTINUOUSLY TUNABLETHIN FILM LASER EMPLOYING THE ELECTRIC FIELD EFFECT OTHER REFERENCESFrench: Franz-Keldysh Effect Light Modulation From Bulk Semi-InsulatingGaAs, IEEE Journal of Quantum Electronics, vol. QE 4, pp 365 6, May,1968 Johnson et al.: Optically Pumped Thin-Platlet SemiconductorLasers," Journal of Applied Physics, vol. 39, pp 3977- Stillman et al.:Volume Excitation of an Ultrathin Single- Mode CdSe Laser, AppliedPhysics Letters, vol. 9, pp 268- 9, October, 1966 PrimaryExaminer-Ronald L. Wibert Assistant Examiner-Edward S. BauerAttorneys-Joseph C. Warfield, Jr., George J. Rubens and John W. McLarenABSTRACT: A selectively tunable thin film laser comprises a source ofexcitation energy and a thin film of laser material positioned tointercept the excitation energy. Conductive means are positioned oneither side of the laser material and insulated therefrom; a source ofselectively variable-electrical potential is connected to the conductivemeans to generate a variable electric field therebetween so as totunably change the wavelength of the laser output.

Patented April 6,1971 3,573,653

FIG. 2

YINVENTOR.

VERN IV. SMILEY BY 1m 2;. QMJMK A TTORNEYS CONTINUOUSLY TUNABLE THINFILM LASER EMPLOYING THE ELECTRIC FIELD EFFECT STATEMENT OF GOVERNMENTINTEREST The invention described herein may be manufactured and used byor for the Government of the United States of America for governmentalpurposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION Known prior art methods of tuning lasershave employed a variable magnetic field, variable temperature, or somemethod of changing the physical cavity size to effect a change in thewavelength of the laser output energy. Most of these known prior arttechniques and methods have been applied to dielectric crystal or to gaslaser's, both of which are of relatively large dimensions of the orderof several centimeters or more in thickness. Relatively very littleknown success has been ac complished in tuning thin semiconductor lasermaterials. Moreover, most of the known thin semiconductor lasertechniques and methods produced a beam of laser output energy parallelto the plane of the thin film of laser material resulting in generallypoor beam properties and limited laser beam aperture size.

BRIEF DESCRIPTION OF THE INVENTION In its simplest form the presentinvention may comprise a thin film or thin slab of semiconductormaterial'in single crystal or polycrystaline form disposed between twothin transparent windows of dielectric material which act as insulators.On the outside of the dielectric or insulating material, a conductivemeans such as a coating or film of conducting material ofsemitransparent character is connected to a source of selectivelyvariable electrical potential. Alternatively, a film of liquid capableof exhibiting desireable lasing action may be used in place of solidlaser material.

This arrangement functions on the principle of the change of index ofrefraction produced along with a change in absorption coefiicientresulting from the application of an electric field to a semiconductoror dielectric lasing material. This effect occurs near an absorptionedge and the change in absorption coefficient is referred to as theFranz-Keldysh effect. A Fabry-Perot optical cavity is formed by thelaser material and the two semitransparent or partially transmittingconductive films. A change in the index of refraction due to the changein the electric field produced between the two conductive materialsbrings about a change in the effective optical thickness of the film,and as a consequence changes the resonant frequency of the opticalcavity. This arrangement will then oscillate as a laser, producing anoutput determinable from the parameters of its operation and is renderedcontinuously tunable by varying the electric field strength. The laserbeam output emerges in a direction perpendicular to the plane of thelaser material and its associated films or coatings. Accordingly, asignificantly greater beam aperture is realized, obviating many of thedisadvantages of prior art arrangements wherein the laser beam emergedfrom the laser material in a direction parallel to its principal axis.

Accordingly, it is a primary object of the present invention to providea continuously tunable thin film laser wherein the wavelength of laserenergy output may be determined from the strength of an electric fieldgenerated across the laser material.

Another most important object of the present invention is to providesuch a thin film laser wherein the effective optical size of the lasercavity may be changed under the control of an electric field.

Yet another most important object of the present invention is to providesuch a thin film laser wherein the effective optical length of the lasercavity is such that only single mode lasing action is generated.

A further most important object of the present invention is to provide acontinuously tunable thin film laser which is significantly smaller insize than known prior art tunable lasers.

A further object of the present invention is to provide a continuouslytunable thin film laser in which the laser energy output emerges in adirection perpendicular to the principal plane of the laser material.

' An ancillary object of the present invention is to provideacontinuously tunable thin film laser which has improved beam qualitiesand beam aperture characteristics.

These and other objects, advantages and features of the presentinvention will be better appreciated from an understanding of theoperation of a preferred embodiment as described hereinafter andillustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a partially cross-sectionalschematic representation of a preferred embodiment of the presentinvention; and

FIG. 2 is-a partially cross-sectional schematic representation ofa'variant embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I illustrates a preferredembodiment of the present invention. A thin film of laser material 10 isdisposed between two transparent windows 11 and I2. The thin film ofsemiconductor laser material 10 may be in a single crystal orpolycrystalline form or comprise a suitable liquid capable of lasingaction. The two transparent windows 11 and 12 are of dielectricinsulating material as well as being transparent to the transmission oflaser excitation energy and laser emission energy. Thin films ofconductive material 13 and 14 are disposed on the dielectricnonconductive transparent films of material II and I2, respectively, onthe sides away from the thin film of laser material I0. Thesecombinations of coatings or successive thin films may each be supportedon an appropriate substrate base such as those shown at 15 and 16. Suchsubstrate or base material as employed at I5 and 16 in the illustrationof FIG. 1 should have good transmission properties relative to thesource of energy which is employed to excite the thin film of lasermaterial 10 to a lasing level, as well as having good transmissionproperties for the wavelength of laser output energy.

Electrical conductors l7 and 18 are disposed in electrical connection tothe films of transparent conductive material I3 and I4, respectively, toprovide electrical paths to the outside of the assembly. A source ofelectrical potential 19, which is preferably selectively variable isconnected through appropriate electrical leads 20 and 21 to theconductors I7 and 18, respectively, so that an electrical potential maybe impressed upon the two semitransparent conductive films l3 and 14 tocreate an electric field thereacross.

A source of excitation energy 22 is disposed so that its output impingesupon the assembly previously described, including the thin film of lasermaterial 10. The excitation energy emanating from the excitation source22 is absorbed in the laser material 10, raising it to a lasing level,whence a beam of laser radiation 23 is generated as an output of theassembly. Preferably, the source of excitation energy 22 is an opticalmeans and may comprise an appropriate noncoherent light source such as agas discharge device or alternatively a source of coherent excitationenergy such as another laser. An optical excitation source of energy isusually of shorter wavelength than the wavelength of the laser energyoutput. Accordingly, the conductive films l3 and I4, as well as thedielectric nonconductive insulating films 11 and 12 and the substratebase 16, must transmit sufficient shorter wavelength excitation energyto produce the desired laser action.

The embodiment illustrated in FIG. 1 operates on the principle of achange of. index of refraction which is produced along with aconcomitant change in absorption coefficient resulting from theapplication of an electric field to a semiconductor or dielectric lasingmaterial. This effect occurs near an absorption edge and, in referenceto the change in absorption coefficient, is customarily referred to asthe Franz-Keldysh effeet.

A change in the index of refraction brings about a change in theeffective optical thickness of the film of laser material and hencechanges the resonant frequency of the Fabry-Ferot opwhere n is the indexof refraction of the laser material, d is the thickness of the laserfilm, o is the central wavelength of its operation, n is the index ofrefraction for the two dielectric insulating films, and 1 is thethickness of each dielectric film.

An electric field strength of 2 l0 V/cm produces an index change of 0.02in GaAs as has been reported by B. O. Seraphin and N. Bottka in AppliedPhysics Letters 6, I34 (1965). This change in index of refraction 8nproduces a wavelength shift of a resonant cavity containing a thin filmof GaAs by about 50 Angstrom units. The device will then oscillate as alaser at a wavelength determined by equation (I) and can be continuouslytuned by varying the electric field strength. The electric fieldstrength is produced by applying a variable voltage source such as thatshown at 19 in FIG. 1 to conductive leads 17 and 18 which areelectrically connected to form a path to the conducting films 13 and 14.The laser output energy emerges in a beam perpendicular to the plane ofthe thin film of laser material as shown in FIG. 1.

The insulating dielectric films l1 and 12 should be thin as possible. Ifa semiconductor film is employed as the laser material in a very thinconfiguration, the insulating material 11 and 12 should also be verythin deposited dielectrics such as MgF L,F, SiO, or other suitablematerials. If a thicker semiconductor film of laser material is employedand disposed generally as shown at 10 in FIG. 1, mica or a similarrelatively thicker insulating sheet of material may be used.

Additionally, in the embodiment of FIG. 1 appropriate antireflectivecoatings may be applied between the laser material 10 and the insulatingmaterials 11 and 12 to reduce loss of effi ciency due to surfacereflection at those two interfaces.

The concept of the present invention contemplates that essentiallysingle mode operation will be employed. This requirement necessitatesthat the optical thickness of the cavi ty be less than a certain valuewhich may be expressed as M 2AA where the expression on the left side ofequation (2) is the total cavity optical thickness, and AA is theemission bandwidth of the laser material employed.

Additionally, the concept of the present invention contemplates that adevice arranged and disposed generally as illustrated in FIG. 1 may beemployed as a tunable amplifier or a tunable filter by causing it tooperate below oscillation threshold and applying an appropriate inputsignal.

As an alternative to the illustrated construction or arrangement, one ofthe substrate materials, preferably the top substrate shown at 15, maybe eliminated so that the conductive dielectric and laser material filmsare deposited directly in the appropriate disposition as taught by theconcept of the present invention.

FIG. 2 illustrates a variant embodiment of the present invention inwhich multiple layers are supported on a single substrate 30. Apartially transparent film 31 of conductive material is depositeddirectly on the substrate base 30. Multiple layers of dielectricmaterial 32, 33, 34, 35, and 36 are deposited over the film ofconductive material 31 and provide electrical insulation between theconductive film 31 and the thin film of laser material 37.

The multiple layers of dielectric material 32, 33, 34,35 and 36 serveanother most important purpose in addition to their insulating function.The layers 32, 33, 34, 35, and 36 are selected to be of alternate highand low index of refraction dielectric materials such MgF and ZnS andeach layer has a thickness substantially equal to one-quarter wavelengthof the emitted laser material. Accordingly, the multiple layers 32, 33,34, 35, and 36 function in the composite as a high reflectance mediumrelative to the emitted laser energy, as well as providing suitableinsulation between the conductive film 31 and the laser material 37.

A second group of multiple dielectric layers 38, 39, 40, 41, and 42 isdeposited upon the thin layer of laser material 37. The layers 38, 39,40, 41 and 42 are characterized by the same alternate high and lowindices of refraction and quarter wavelength thicknesses as thepreviously described layers 32, 33, 34, 35, and 36.

A conductive film 43 is deposited over the multiple layers 38,39,40,41and 42 and conductors 44 and 45 are provided to make electrical contactwith the conductive films 31 and 43, respectively. The conductive films31 and 43 are preferably recessed from the edges as illustrated toprovide adequate insulation effect. Suitable electrical leads 46 and 47connect the conductors 44 and 45, and their respectively associatedconductive films 31 and 43, to a source of selectively variableelectrical potential 48.

In operation, a source of appropriate excitation energy 49 is directedat the assembly as illustrated in FIG. 2, raising the laser material toa lasing level and generating laser output beams as shown at 50, both ofwhich emerge perpendicular to the principal plane of the thin film oflaser material 37. Upon selective change of the electrical potential 48applied to the two conductive films 31 and 43, a commensurate changeresults in the electric field between the conductive films. The changein electric field causes a consequent change in the effective opticalthickness of the laser material and a consistent change in thewavelength of the energy emitted by the laser material.

It will be appreciated by those skilled and knowledgeable in the artthat the concept of the present invention provides a laser which offersmarkedly improved facility of tunability and is capable of producing awider range of wavelengths of output laser energy than was heretoforeknown to be possible. Moreover, the laser of the present inventionproduces a laser output beam which emanates in a direction perpendicularto the plane of thin film of laser material, obviating several of thedisadvantages of poor beam properties and limited aperture size whichwere inherent in some prior art laser arrangements.

An additional highly desirable advantage of the present invention whichis inherent in its concept, is that is provides a laser with manydesirable characteristics and inherently is of smaller size and morecompact in its arrangement than many prior art lasers. y

In the described preferred embodiment employed for purposes ofillustration and explanation it should be appreciated that theillustrations of FIGS. 1 and 2 are not drawn to scale in the interestsof simplicity and clarity in understanding their operation. Thoseskilled and knowledgeable in the art will appreciate that the extremelythin films and coatings referred to in the foregoing explanations are ofthe order of a relatively few wavelengths of the energy involved andthat the proportions shown in the illustrated embodiments are notintended to be scalar representations.

Moreover, the laser material may comprise a thin film of solid materialor a suitable thin film of liquid lasing material.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. A tunable thin film laser comprising: an external source ofexcitation energy for pumping said laser;

a thin film of laser material positioned in the path of said excitationenergy having a thickness d and an index of refraction n parallel thinfilms of dielectric material having a thickness 1 and an index ofrefraction n positioned on opposite sides of said laser material;

a semitransparent conductive film positioned on the outside of each saidfilm of dielectric material to form an optical cavity having a thickness(n, d+2n l which is no greater than A /2AA where A is the centralwavelength of the laser operation and AA is the emission bandwidth ofthe 5 laser material a source of selectively variable electricalpotential; and means connecting said source of electrical potentialacross said conductive films for generating a variable electric fieldtherebetween to vary the index of refraction near the absorption edge ofsaid laser material, whereby to tunably change the laser outputwavelength.

2. A tunable thin film laser as claimed in claim 1 wherein said thinfilm of laser material is a semiconductor.

3. A tunable thin film laser as claimed in claim 1 wherein saidexcitation energy is a source of optical radiant energy.

4. A tunable thin film laser as claimed in claim 1 wherein saidconductive and dielectric films transmit a substantial portion of saidexcitation energy.

5. A tunable thin film laser as claimed in claim 1 wherein the effectiveoptical thickness of the cavity formed by said film of laser materialand its covering films is such that only single mode lasing action isgenerated.

6 A tunable thin film laser as claimed in claim 1 adapted to accept aninput signal as a tunable amplifier by operating below its oscillationthreshold.

7. A tunable thin film laser as claimed in claim 6 wherein said inputsignal comprises optical energy within the wavelength region of thelaser energy output of said thin film of laser material.

2. A tunable thin film laser as claimed in claim 1 wherein said thinfilm of laser material is a semiconductor.
 3. A tunable thin film laseras claimed in claim 1 wherein said excitation energy is a source ofoptical radiant energy.
 4. A tunable thin film laser as claimed in claim1 wherein said conductive and dielectric films transmit a substantialportion of said excitation energy.
 5. A tunable thin film laser asclaimed in claim 1 wherein the effective optical thickness of the cavityformed by said film of laser material and its covering films is suchthat only single mode lasing action is generated. 6 A tunable thin filmlaser as claimed in claim 1 adapted to accept an input signal as atunable amplifier by operating below its oscillation threshold.
 7. Atunable thin film laser as claimed in claim 6 wherein said input signalcomprises optical energy within the wavelength region of the laserenergy output of said thin film of laser material.